Britain leaving the EU would 'put the economy at risk,' business leaders are set to warn as they partner with David Cameron to step up their campaign against Brexit.

Bosses from up to a half of firms in the FTSE-100 are expected to sign a letter praising the deal reached by the Prime Minister after marathon talks in Brussels last week.

Tory party officials started cobbling the letter together on Thursday - before Mr Cameron's EU deal had even been finalised with European leaders.

Bosses from up to a half of firms in the FTSE-100 are expected to sign a letter praising the deal reached by the Prime Minister after marathon talks in Brussels last week

Backbench Tory MPs said it was proof that even Downing Street viewed the renegotiation as 'irrelevant' and nothing more than 'imaginary gruel'.

Downing Street is believed to be helping with drawing up the final letter, which is likely to spark further accusations of an orchestrated attempt in Whitehall to scaremonger voters - similar to the criticism the UK Government received before the Scottish independence referendum.

It comes on a day when the referendum campaign kicked off in earnest, with Boris Johnson electrifying the EU referendum campaign by declaring his support for Britain to leave the EU.

It is a major blow for the Prime Minister, who made a last ditch attempt to persuade him to join the In camp, warning him against 'linking arms with Nigel Farage and George Galloway and taking a leap into the dark'.

Advertising maestro Sir Martin Sorrell is one of the names expected to appear on the letter warning about the economic dangers of Brexit

Earlier this morning Mr Cameron declared war on his own Eurosceptic Cabinet ministers by accusing them of misleading the public over claims that leaving the EU would win back control over immigration.

Pro-Brexit Cabinet ministers challenged the Prime Minister's EU deal today and Iain Duncan Smith, the Work and Welfare Secretary, directly contradicted Mr Cameron over claims that Britain is 'safer and stronger' in the EU.

He warned that remaining a member of the EU would make a Paris-style terror attack in the UK more likely.

'This open border does not allow us to check and control people that may come and spend time,' he told the BBC today.

'We've seen what happened in Paris where they spent ages planning and plotting so who's to say it's not beyond the wit of man that those might already be thinking about that.'

Asked by the BBC whether staying in the EU made the UK more susceptible to Paris-style attacks, he replied: 'I think the present status of the open border we have right now many of us feel does actually leave that door open and we need to see that resolved.'

The letter from business chiefs - expected to be published on Tuesday, is expected to say: 'We run businesses representing every sector and region of the United Kingdom.

'Together‎ we employ hundreds of thousands of people across the country.

'Following the Prime Minister's re‎negotiation, we believe that Britain is better off staying in a reformed European Union.

'He has secured ‎a commitment from the EU to reduce the burden of regulation, deepen the single market and to sign off crucial international trade deals.

Iain Duncan Smith, pictured left leaving the historic Cabinet meeting yesterday, contradicted the Prime Minister, pictured rightleavnig the BBC studios today, by saying staying in the EU would make Britain more exposed to Paris-style terror attacks

Boris Johnson, pictured arriving at his London home this afternoon, has decided to join the Out campaign, dealing a major blow to David Cameron's hopes of staying in the EU

'Businesses like ours need unrestricted access to the European market of 500m people in order to continue to grow, invest and create jobs.

'We believe that leaving the EU would deter investment and threaten jobs.

'It would put the economy at risk.

'Britain will be stronger, safer and better off remaining a member of the European Union.'

SOUND-BITE DAVE: VOTERS WARNED FOR AVALANCHE OF KEY SLOGANS FROM PM

David Cameron, pictured on the Andrew Marr Show this morning, said THE slogan ‘safer, stronger, better off’ nine times during his interview

Voters have been warned to expect a four month campaign of soundbites as David Cameron repeatedly uses a handful of key phrases to try and convince people the country would be less safe if we left the EU.

In the 48 hours after being handed his renegotiation deal in Brussels, the former PR man deployed the carefully crafted soundbites over and over again as he hit the airwaves to set out his referendum stall.

During a 20-minute interview on the BBC’s Andrew Marr Show today the Prime Minister three times warned that a vote to leave would be a ‘leap in the dark’.

Mr Cameron also used the phrase, which is expected to be a cornerstone of the In campaign that has been nicknamed ‘Project Fear’, in a speech in Brussels on Friday night and again the next morning in Downing Street as he announced the date of the referendum.

In the three appearances, which were all televised, Mr Cameron used the slogan ‘safer, stronger, better off’, nine times – including twice in the address outside Number 10 that lasted only four minutes.

He separately used the words ‘safe’ and ‘safer’ another eleven times.

Despite failing to secure the curbs to migrant benefits promised at the election in the Tory manifesto, Mr Cameron repeated the expression ‘something for nothing’ six times as he attempted to sell the changes he did get that will stop newcomers being able to claim tax credits from day one.

Mr Cameron also delivered the catchphrase ‘the best of both worlds’ six times and ‘strength in numbers’ three times.

In a round of television and radio appearances at breakfast time on Saturday, Chancellor George Osborne continually used the phrases as well.

During the last parliament in the run up to last year’s election, Mr Cameron deployed the phrase ‘long-term economic plan’ so many times it became a standing joke in Westminster.

Mr Cameron rarely resisted the opportunity to shoehorn several mentions of it into his appearances at Prime Minister’s Questions.

By repeating his referendum soundbites ad nauseum, Mr Cameron is no doubt hoping to get the messages lodged in the public’s consciousness in the run up to the vote on 23 June.

It is thought the short sharp messaging will have been tested in focus groups.

NOW YOU DECIDE: YOUR AT A GLANCE GUIDE TO HOW JUNE 23 REFERENDUM WILL WORK

David Cameron, pictured on the Andrew Marr Show this morning, said holding the EU referendum was an act of sovereignty in itself as he hailed his EU deal reached with European leaders late on Friday evening

With the opinion polls see-sawing – and up to 40 per cent of voters saying they could change their mind between now and June 23 – the EU referendum is up for grabs.

Harold Macmillan famously declared that 'events, dear boy, events' were the biggest threat to a leader's plans. Now, another Conservative Prime Minister will be hoping the historic vote is not decided by factors beyond Downing Street's control...

MIGRATION

Last summer's migrant crisis, which saw hundreds of thousands of Syrians and North Africans making the perilous journey by boat to Europe, triggered a boost in the polls for the Out camp.

Reports that the same migrants were responsible for sex attacks in Germany, while others were living in the 'Jungle' camp at Calais, increased the impact.

Last year, an astonishing 1.83 million people illegally entered the EU, over six times as many as the previous year.

This is why Downing Street was so keen to hold the referendum in June. The next possible date was after the summer, by which time the migrant situation could have deteriorated even further.

This year's crisis is predicted to be even more serious than last year. If so, it could hand victory to the 'Out' campaign.

JOBS

David Cameron needs Chancellor George Osborne – who is also the Government supremo trying to engineer a win for the 'In' campaigners – to deliver a feelgood Budget in May. If voters are positive about their personal financial circumstances, they are less likely to vote for change.

More important will be the performance of the wider EU economy. A repeat of the eurozone crisis of two years ago, when Greece and other southern European governments were on the brink of collapse, would reinforce the notion that Britain's economy would thrive outside the bloc – and with it a burst of job creation.

'In' campaigners will insist that British jobs are heavily tied to our trade links with our EU partners.

In a speech yesterday, Labour leader Jeremy Corbyn said: 'Being part of Europe has brought Britain investment, jobs and protection for workers.'

But 'Out' campaigners will highlight sluggish growth rates across the EU as a reason to break free.

SECURITY

Downing Street is planning to claim that if Britain leaves the EU, we will be less protected against threats such as Putin's Russia because hostile powers are intimidated by the unity of the 28-state EU. 'Out' campaigners will counter this argument by highlighting the security risks presented by lax border controls.

The terror attacks in Paris last November accentuated these fears, particularly after it was revealed that some of the perpetrators had posed as migrants to enter Europe.

Home Secretary Theresa May has argued that being part of the EU is a major benefit to British security services as a result of cross-border intelligence sharing and valuable reciprocal agreements such as the European Arrest Warrant.

SOVEREIGNTY

Cameron has tried to win over London Mayor Boris Johnson to the 'In' campaign by promising to enshrine in British law a pledge guaranteeing the ultimate supremacy of Parliament over Brussels – and hopes voters will also be persuaded.

The Prime Minister was today expected to promise to make clear that the British Supreme Court outranks the European Court of Justice, similar to an idea first put forward by Johnson last year as the price for his support for the 'In' group.

He says it is critical the sovereignty of the House of Commons is 'put beyond doubt'.

Legal experts say the move is pointless as Parliament already has the power to ignore EU law if it chooses to, it is just that the sheer volume of Brussels legislation makes it impractical – but it may have some impact on voters' sentiment due to the 'political theatre' it represents.

INFLUENCE

The Prime Minister is planning to make British influence on the world stage a key strand of his campaign – arguing that we have far more clout combined with our EU partners, rather than as a 'lone wolf'. It explains the slogan of the 'In' campaign group: 'Britain Stronger in Europe.'

The arguments will be played out whenever a diplomatic crisis strikes, such as attempts to settle the war in Syria, but are likely to pivot on economic considerations.

The 'In' camp says that Britain is better able to open up new trade markets by acting with other EU countries. The 'Out' camp says British businesses are dragged down by Brussels' red tape and would flourish by forging fresh alliances within the Commonwealth.

They say our influence within the EU is illusory because we are routinely outvoted on important decisions in Brussels, winning only eight per cent of the votes on vital EU decisions.

RISK

The 'In' camp believes its trump card is 'Project Fear' – making sure voters are too frightened to make a 'leap in the dark' and instead plump for the security of the status quo.

They will say we cannot risk being cut off by our EU partners.

'Out' campaigners will argue the UK is risking its future by being subservient to Brussels and make reassuring noises about life outside the EU. As leading 'Outer', Cabinet Minister Chris Grayling said yesterday: 'Does anybody actually think that on the day after Britain leaves the EU, the Germans are going to turn around and say, 'We're not going to sell you BMWs any more'?

AT 12.23 PM SAYS 'I'M IN', BUT AT 13.05 GANG OF SIX USE BANNER TO PARADE THEIR DEFIANCE

As soon as yesterday's special Cabinet meeting had concluded, Michael Gove slipped out of the back entrance of Downing Street and headed for a rendezvous with the other five 'Outers'. At the headquarters of the Vote Leave group, based in a tower block on the south bank of the Thames, the Justice Secretary posed with his fellow 'freedom fighters' – as they have been dubbed by admirers – as they clutched a signed campaign banner. They are:

1) JOHN WHITTINGDALE

The Culture Secretary is the last authentic Thatcherite in the Cabinet: in the 1980s he worked as her private secretary and has remained loyal to her ideology – including her Euroscepticism – since. An unlikely-seeming Meatloaf fan, Whittingdale was a tenacious chairman of the Commons Media Committee, where he subjected figures such as Rupert Murdoch to forensic interrogation. He is said to be thoroughly enjoying his portfolio, which includes a root-and-branch review of the BBC's funding.

2) THERESA VILLIERS

The Northern Ireland Secretary is a Major's daughter who is descended from Edward I. Along with Chris Grayling, the Commons Leader, she is understood to have spent the past six months agonising over whether to leave the Cabinet to campaign for an 'Out' vote. However, after a chat with Mr Cameron before Christmas, she was reassured she would be allowed to campaign from within the Cabinet – sparing the PM an awkward reshuffle. Her stance has caused controversy in Northern Ireland, with local politicians claiming an EU exit could stoke sectarian tensions.

3) MICHAEL GOVE

The Justice Secretary is the star name out of the six – and the one with the closest personal relationship to the Prime Minister. An ex-Times newspaper executive and author, who has been friends with Mr Cameron for more than a decade, he was part of the 'kitchen cabinet' who encouraged him to run for the Tory leadership. Throughout his Cabinet career, Gove has been Mr Cameron's first choice to help him prepare for the weekly ordeal of PMQs, amusing No 10 staffers with his razor-sharp rejoinders in practice sessions.

Gove was a controversial reforming Education Secretary until moved to the Whips' Office after private party polling suggested he was an electoral liability.

The move caused a 'wobble' in his and Mr Cameron's friendship.

Mr Gove said he had wrestled with his conscience before announcing he would back Out but later posed with Vote Leave activists having deciding to make the leap away from his old friend Mr Cameron

4) CHRIS GRAYLING

Mr Gove's predecessor as Justice Secretary, Chris Grayling is a long-term Eurosceptic. The Manchester United fanatic was deployed by the Tories as an energetic 'attack dog' during their long period in Opposition, harrying Labour – including the Blairs – over their personal business affairs. The 6ft 5in Leader of the House has a cool relationship with Gove, who has reversed many of his justice reforms.

5) IAIN DUNCAN SMITH

Few people were surprised by Iain Duncan Smith's decision to back the Out campaign: the Work and Pensions Secretary has been a long-standing critic of the EU and supporter of tighter immigration controls. The former Scots Guards officer's undistinguished period as Tory leader between 2001 and 2003 is remembered largely for the conference speech he gave in which he portrayed himself as 'a quiet man' who was 'turning up the volume'. Duncan Smith has since rebuilt his reputation by becoming a crusading campaigner for social reform, which prompted David Cameron to put him in charge of the Government's welfare shake-up.

6) PRITI PATEL

Priti Patel, who answers to Mr Duncan Smith as Employment Minister, is regarded by the former Tory leader's team as the up-and-coming poster girl for Cabinet Eurosceptics. As the daughter of Ugandan immigrants who came to this country in the 1970s to seek sanctuary from Idi Amin, she says that the welcome they received here has instilled in her a fierce British patriotism. Patel, whose political heroine is Margaret Thatcher, started out in her Westminster career working for Sir James Goldsmith's Eurosceptic Referendum Party in the 1990s, and sees her current stance as being consistent with her long-term political beliefs.